This invention relates to masks used for charged particle lithography. It relates more specifically to electron lithography systems employing thin membrane masks used to scatter electrons.
Masks for charged particle lithography, in particular electron beam projection lithography (EPL), are formed from thin membrane masks. It has been found that if the membrane or scattering material has some intrinsic stress and is patterned, the placement of the images distorts as the stress is relieved due to removal of the stressed layers. This effect is largest when a large gradient exists in the pattern density, such as half of the membrane being patterned with a dense pattern and the other half being unpatterned. This leads to a large membrane distortion that is impossible to correct in the e-beam optics. Patterning the entire membrane also leads to a large distortion, but this can be corrected by a magnification correction in the e-beam optics.
This problem is illustrated by FIGS. 1 and 2. FIG. 1 shows a schematic of a half-patterned region on a membrane. FIG. 2 shows a corresponding image placement distortion pattern for a 1×1 mm stencil mask. See Lercel et al., J. Vac. Sci Technol. B. 19(6), pp. 2671–2677 November/December (2001) for a further explanation of pattern-induced image placement distortions.